1. Field of the Invention
This invention is related to transmitting ceramic materials and, more particularly, to ternary sulfides useful as infrared materials such as in windows and missile domes. This invention also relates to a new synthesis of metal sulfides.
2. Description of the Prior Art
Infrared transmitting materials particularly for the 8-12 .mu.m region (1 .mu.m is equal to 10.sup.-6 meters) are necessary for sensor windows and domes ons taellites, missiles and other similar devices. These materials must be chemically stable, abrasion resistant, have low coefficients of thermal expansion, high melting temperatures and good optical transmission characteristics in the desired region of the infrared.
There are known methods for synthesizing metal ternary sulfides. In one method ternary sulfides were prepared in powder form by reacting carbonates of the alkaline earth elements with either oxides or hydroxides of the lanthanide elements at 1000.degree. C. in an atmosphere of flowing H.sub.2 S. The starting material sin powder form were mixed in the correct stoichiometric ratio, placed in boats of pyrolytic graphite and inserted into silica-glass furnace tubes. Typical reaction times were from 3 to 7 days. With this ethod, however, it was found difficult to achieve the exact stoichiometry required by the ternary sulfide compounds. Moreover, the end product contained CaS as an impurity.
In another method, sulfides of the general formula M.sub.2 M'S.sub.5 where M may be lanthanum and M' is one or more of hafnium and zirconium, are prepared by direct combination from the elements or from precursor compounds. Combining of the elements generally occurs by solid state diffusion, and times and temperatures appropriate to such reactions are needed. In general temperatures in the range of about 500.degree. C.-1200.degree. C. are used for times ranging from about four hours to four days or more, preferably in several stages with intermediate grinding to promote intimate contact between reactants.